What is the difference between P0203 and a misfire code like P0303?

P0203 is a specific electrical open circuit on cylinder 3's injector. P0303 simply says cylinder 3 is misfiring for any reason. The two can appear together when the dead injector causes the misfire.

Could low fuel pressure cause P0203?

No. Low fuel pressure affects all cylinders and sets pressure or lean codes. P0203 is isolated to the cylinder 3 injector circuit and is electrical in nature.

Is it worth cleaning the injector?

Cleaning will not cure an open circuit. If the coil has failed electrically the injector must be replaced; ultrasonic cleaning only helps with blockage, which is a different problem.

Will the car pass an MOT with P0203?

No. A flashing engine light or an active emissions-related fault fails the MOT, and a misfiring cylinder pushes emissions out of limits as well.

How long does an injector swap take?

On a petrol engine with accessible injectors, often under an hour. Diesel jobs take longer and add coding time, especially if the injector is seized in the head.

Can corrosion alone trigger P0203?

Yes. Enough corrosion in the connector raises resistance until the circuit reads as open. Cleaning or replacing the connector can fix it without touching the injector.

P0203 - Injector Circuit Open - Cylinder 3

What Is P0203?

P0203 means the ECU has detected an open circuit on the cylinder 3 fuel injector. The firing signal cannot reach the injector, so it stays shut and cylinder 3 is starved of fuel while every other cylinder keeps working normally.

The result is a hard, lopsided misfire and a flashing warning light under load. Unburnt charge passing through cylinder 3 skews the oxygen sensor and threatens the catalyst, so although the car will limp along it should not be driven far in this state.

Common Symptoms

Persistent misfire that worsens under acceleration
Engine management light, often flashing
Power loss and flat throttle response
Rough, uneven idle
Petrol or eggy exhaust smell
Increased fuel consumption

Common Causes

Failed solenoid coil - an open winding inside the cylinder 3 injector is the most likely root cause.

Loose or broken connector - the injector plug working loose or its locking tab snapping off.

Wiring break - an open in the short harness branch feeding cylinder 3, frequently from heat or vibration.

Pin corrosion - oxidised pins in the connector creating an effectively open joint.

Supply fault - a blown fuse or failed relay cutting power to the injector circuit.

Internal ECU fault - a dead injector driver channel in the ECU, rare but not impossible.

How to Diagnose P0203

Locate Cylinder 3

Counting from the cam-belt end, cylinder 3 is the third injector along. Confirm the layout for your engine so you test the correct unit.

Check for a Firing Click

A long screwdriver or stethoscope against the injector body picks up the rapid click of a working injector. No click on cylinder 3 backs up the open-circuit reading.

Ohm Out the Injector

Disconnect and measure the injector's resistance. Open circuit (infinity) confirms a failed coil; a normal reading (roughly 12–16 ohms petrol, 0.5–2 ohms diesel) clears the injector and points at the wiring.

Inspect the Connector Closely

Look for spread pins, corrosion, melted plastic, or a loose seal. Reseat firmly, clear the code, and see if it returns - intermittent connectors are common.

Test the Harness Branch

With everything unplugged, check continuity from the connector back to the ECU pin. Wiggle the loom while testing to catch a break that only opens when warm or vibrating.

Confirm by Substitution

Move the cylinder 3 injector to another cylinder. If the fault follows the injector the part is bad; if it stays on cylinder 3 the wiring or ECU is to blame.

Do Not Keep DrivingRunning on three cylinders forces unburnt fuel into the catalytic converter and can overheat it. Limit driving to getting the car somewhere it can be fixed, and avoid sustained high revs.

Verdict

Prove the injector first with a resistance check. An open coil on cylinder 3 is the common answer; if the injector tests fine, focus on the connector and the harness branch back to the ECU before replacing parts.

Want the full picture? The OBD Fault Code Plain English Guide (PDF) covers the most common UK fault codes in one plain-English download.